1. Field of the Invention
The present invention relates to a camera, and more particularly to a method and apparatus for auto-focusing a camera by utilizing a distance measuring sensor and an encoder.
2. Description of the Related Art
Conventional cameras include a lens system for forming images of subjects on film or an image sensor. An image sensor detects the images formed by the lens system and converts them to electrical signals. In either case, the film or the surface of the image sensor is an image plane. The focus of the lens system varies its position in accordance with distances between the lens and the subjects. Thus, pictures of good quality focus can only be obtained when an amount of variation of the position of the image plane with respect to the position of a subject is within the depth of focus of the camera. That is, in order for the camera to obtain clear images, the light receiving plane of the image sensor should be positioned within the depth of focus of the lens system.
Common cameras, especially cameras with a macro function which has a great variation of focal position with respect to variation of the distance to the subject, i.e. cameras with a close-up function, should be provided with a device capable of moving the position of the lens to account for the distance to the subject. Cameras that automatically adjust the relative position are called Auto Focus (AF) cameras. In order to play a leading role in multimedia communication today, it may be essential for cameras to have such an AF function.
Controlling the AF function of a camera may be typically divided into a passive method and an active method according to a measurement manner of the distance between the camera and the subject.
The passive method obtains edge values appropriate for adjusting focus by detecting high frequency components of an image obtained from the image sensor and moving the lens to a position with a maximum edge value. A driving unit moves the focus lens step by step in response to a control signal of a controller to obtain the maximum edge value. Since this passive method allows for obtaining information on focus control from the image signal without additional devices and has high accuracy while allowing a small size device, it is widely applied to most cameras. An inner configuration of a camera using the passive method above is described with reference to FIG. 1.
FIG. 1 is a view illustrating a prior art inner configuration of a camera with the passive auto-focusing function.
Referring to FIG. 1, the camera with the passive auto-focusing function includes a lens 101 for forming an image of an subject, a focus lens 103 for accurately focusing the formed image, an image sensor 107 for detecting the formed image as electric signals, a driving unit 109 for moving the focus lens, the ISP (Image Signal Processor) 111 for processing an image signal input from the image sensor and a driving current per frame unit, and the controller 113 for controlling the driving unit 109.
With reference to the above configuration, a method for obtaining edge values and moving the focus lens 103 to a position with a maximum edge value is described below.
FIG. 2 is a graph illustrating a prior art variation of edge values according to positions of the lens of the camera with the conventional passive auto-focusing function.
Referring to FIG. 2, a movement distance of the focus lens for performing the auto-focusing is divided into 10 sections, from 0 to 9, according to the number of steps. The edge values for every step is detected while moving the focus lens sequentially from step 0 to step 9. If step 5 has the maximum edge value of the detected edge values as shown in FIG. 2, then the auto-focusing function is achieved by moving the focus lens again to a position which has a fifth edge value corresponding step 5.
According to the active method, a distance measuring sensor emits infrared rays, ultrasonic waves, lasers, or any other type of radiation suitable to measure distances toward the subject in order to measure the distance between the camera and the subject. The active method advantageously works effectively in the dark, but requires additional hardware such as the distance measuring sensor to emit and detect signals. An inner configuration of the camera with the active auto-focusing function is described below with reference to FIG. 3.
FIG. 3 is a view illustrating an inner configuration of a camera according to a prior art conventional active auto-focusing function.
Referring to FIG. 3, the camera with an active auto-focusing function includes a lens for forming an image of an subject, a focus lens for accurately focusing the formed image, a driving unit for moving the focus lens, the ISP for processing an image signal input from an image sensor and a driving current per frame unit, and a distance measuring sensor for measuring a distance between the camera and the subject, etc.
Assuming that a light emitted from the distance measuring sensor is an infrared ray, the infrared ray emitted from a transmitter Tx of the distance measuring sensor is reflected from the subject and detected by a receiver Rx of the distance measuring sensor. This obtains the distance L between the camera and the subject. The obtained value is transferred to the ISP. The ISP receiving the distance value L determines, through a predetermined algorithm, a displacement x that the focus lens has to be moved. The displacement x can be represented by equation (1) below.x=f(L)  (1)
Equation (1) above represents the displacement x when the distance between the camera and the subject is L. The auto-focusing Equation (1) is described below with reference to FIG. 4.
FIG. 4 is a prior art graph illustrating position variations of focus lenses according to inputs, such as currents or voltages, transferred to driving units of the cameras with the active auto-focusing function. Variations in accordance with the respective driving units are represented by A, B, and C.
Referring to FIG. 4, due to characteristic variation of driving units, i.e. characteristic variation according to elements constituting a voice coil motor (VCM), for example, a spring constant k, the number of winding turns of the coil, mechanical interference, etc., initial driving current values for moving the focus tenses to a position where the subject is in complete focus, i.e. “x”, and the slopes of A, B, and C at “x” are different from each other.
As described above, the passive and active methods are used differently according to purposes and characteristics thereof. The driving units employed by each method are both manufactured in order to increase manufacturing yield such that initial driving currents to start the driving units and slopes according to the currents are kept within respective predetermined ranges.
However, when AF cameras are manufactured in the above way to increase manufacturing yield, instead of applying accurate driving currents to corresponding driving units, a minimum current value of the ranges determined for the process' sake is collectively applied to all driving units. Therefore, a common driving current is applied to the driving units, even though each driving unit may have different driving current values of their own. The applied current value is not the most suitable to the respective driving units. Accordingly, in some cases of auto-focus modules, there are auto focus steps which are actually not used therein, thereby increasing auto-focusing time thereof. Besides this common problem, the above passive and active methods have their respective problems.
A problem with the passive method is since the focus lens is positioned at the step having the maximum edge value after it is moved step by step to detect edge values, auto-focusing time is relatively long because each step must be undertaken. Additionally, due to this relatively time consuming process, it is difficult to successively perform auto-focusing when taking moving pictures, and thus quite difficult to obtain clear moving images, resulting in customer dissatisfaction.
The active method has a problem that individual algorithms are required for the respective driving units in order to move the focus lenses to their positions having maximum edge value since each driving unit has a different displacement value according to characteristics of manufacturing processes. Such algorithms themselves are difficult to implement, and implementing the individual algorithms increases manufacturing cost.
In order to improve the problems above, an auto-focusing method is required that can ensure a fast operation characteristic of a camera, irrespective of driving currents of driving units.